This paper investigates the quantization methods of the reliability-based iterative decoding algorithm for LDPC codes. For the uniform quantization, we present a new design criterion to determine the quantization parameters with the clipping interval/probability. Compared to the original parameters setting, the presented design criterion can achieve higher resolution with the allowable quantization distortions. The presented parameter design criterion can be extended to the non-uniform quantization with a slight modification. Simulation results show that, the decoding algorithm can achieve better BER performances and faster convergence rate with smaller quantization bit when compared to the original parameters setting.